Given the fact that all languages variably differ at phonology, vocabulary, grammar (syntax), and discourse levels. I think of learning language through instruction rather than acquisition. It's believed that the nationals whose first languages ( mostly of latin origin) are close to the English language excel those whose first languages are far off (Arabic, Persian, Japanese, and many others). Symptoms of foreignized language primarily reveal themselves through accent, though it makes up only a small part of the language. For example, the Japanese, Chinese, Iranians, French ... speak English with a special type of accent which reveal their true identities.
In addition to the linguistic factors mentioned above, Language learners come up with affective factors which many believe are more important than linguistic ones. Such personal factors as motivation, attitude, interest, and the effort the learners put into learning the language greatly affect learning and lead to learners who amazingly improve their four language skills.
From your experience of teaching different nationals, what do you think? Note that all nationalities are respectable. Please avoid comments that may be offensive to certain ethnic groups.
bye the chance i am iranian consequently i have commuincated with cornuopia of people and students who have been studying english.however,it seems difficult they have surpassed the obstacles and they are shining in english in spite the fact that sadtosay our country is concealed under political shadows and there is not any special cmmunication for our people to another foriegn people.therefore i can exemplify my people as one of smartest nations in the world.in hope of having more global communication and advocation.
From purely anecdotal evidence, I think many people from Sweden and Norway have an extraordinary facility with English. This is probably due to their exposure to the language from an early age (TV programmes, I'm told), the phonological similarities between English and their mother tongues, and perhaps their language lessons at school. Many of the students I've taught from these countries were approaching native-speaker competence.
Again, purely anecodotally, I've particularly enjoyed working with Korean students. From my experience they're ideal students. They're hard working, conscientious and also seem willing to speak and don't seem too afraid to make mistakes.
Of course, these qualities can be found anywhere and as you point out we must be careful not to generalise (although generalising can be fun and helps us deal with the world) but when I saw this question I immediately thought of Koreans.
... and further to this idea of generalising, I can't remember where this quote comes from - and I might be paraphrasing - but I think it's fantastic: "Your prejudices are the windows through which you views the world; just take care to clean them once in a while."